1. Field of the Invention
The present invention relates to a fire control system comprising at least a target sensor and computing means coupled to it, the target sensor being arranged for the detection of targets such as aircraft and for the provision of signals p(t) related to measured target positions such as measured target positions, the computing means being arranged for the computation of an estimated frequency-determining value w as well as an estimated amplitude A of a periodic component of target signals s(t) derived from the signals p(t) and associated with a target state such as estimated positions, velocities or accelerations of the target.
The invention also relates to a method of estimating a frequency-determining value w and an amplitude A of a periodic component in target signals s(t) derived from measured positions of a target such as an aircraft and associated with a target state such as estimated positions, velocities or accelerations.
In this case, the frequency-determining value w can comprise either a frequency or a period of the target signals. The target signals s(t) may comprise a linear combination of estimated or measured target positions, target velocities or target accelerations.
2. Discussion of the Background
Fire control systems and methods of this type are known. It is often necessary, for instance with a view to the calculation of the lead angle for a gun system connected to the fire control system, to be able to make a prediction of values related to the target path. These values include not only predicted target positions, but also target velocities or target accelerations. A target can be, for instance, an aircraft or a missile performing evasive manoeuvres. In such a case, as appears from actual practice, aircraft often follow a weaving path. If the period or frequency and the amplitude of the weaving path are known, it is feasible to make an accurate estimate of target states, such as target positions or target velocities, in the immediate future. In actual practice however, neither the frequency nor the amplitude of the path is known. Notwithstanding this, it is possible to proceed on the basis of an estimated frequency and amplitude.
Fast reaction is often mandatory to fire control systems. Therefore, within a short space of time and on the basis of a limited amount of information, an accurate estimate of the target path has to be obtained. The Kalman filtering techniques known in the art do by themselves not offer a satisfactory solution to this problem, since Kalman filters invariably take a certain time to settle. If the target follows a path in a weaving pattern whose frequency is continuously changing, a Kalman filter will respond too slowly and will not immediately generate an accurate new estimate of both the frequency and the amplitude for the target path. This is partly caused by the fact that a Kalman filter always lets a new estimate of both the frequency and the amplitude depend on preceding estimates of the frequency and the amplitude. However, it appears to be feasible to estimate with great precision the frequency continuously independently of the amplitude. On the basis of the estimated frequency, an accurate amplitude estimate can subsequently be made. By this method, an accurate estimation result is sooner obtained than with the application of a Kalman filter.